Based Control

Up to this point we have usually chosen a type of controller (P, PI, or PID) and determined the tuning constants that gave some desired performance (closedloop damping coefneient). We have used a model of the process to calculate the controller settings, but the structure of the model has not been explicitly involved in the controller design. 

There are several alternative controller design methods that make more explicit use of a process model. We will discuss two of these below. 

This method originated in sampled-data control and we will discuss it in that context in Chap. 20. However, the same thinking can be applied in continuous systems. 

In this approach, the desired closedloop response for a given input is specified. Then, knowing the model of the process, the required form and tuning of the feedback controller is back-calculated. These steps can be clarified by a simple example. 

Example 11.7. Suppose we have a process with the openloop transfer function 

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An ethylene plant contains more than 300 equipment items. Traditionally, operators were trained at the site alongside experienced co-workers. With the advent of modem computers, the plant operation can be simulated on a real-time basis, and the results displayed on monitors (107). Computers are used in a modem plant to control the entire operation, eg, they are used to control the heaters and the recovery section (108). A weU-controUed plant is much more profitable than a poorly controlled plant. For the heaters, a model-based control system is gaining importance (109). Instead of simply controlling the coil outlet temperature (COT), severity is actually controlled. The measurement of severity (either or C H /CH ratio) requires on-line effluent... 

The Smith predictor is a model-based control strategy that involves a more complicated block diagram than that for a conventional feedback controller, although a PID controller is still central to the control strategy (see Fig. 8-37). The key concept is based on better coordination of the timing of manipulated variable action. The loop configuration takes into account the facd that the current controlled variable measurement is not a result of the current manipulated variable action, but the value taken 0 time units earlier. Time-delay compensation can yield excellent performance however, if the process model parameters change (especially the time delay), the Smith predictor performance will deteriorate and is not recommended unless other precautions are taken. 

Introduction The model-based contfol strategy that has been most widely applied in the process industries is model predictive control (MFC). It is a general method that is especially well-suited for difficult multiinput, multioutput (MIMO) control problems where there are significant interactions between the manipulated inputs and the controlled outputs. Unlike other model-based control strategies, MFC can easily accommodate inequahty constraints on input and output variables such as upper and lower limits or rate-of-change limits. 

Electronic Controllers Almost all of the electronic process controllers used todav are microprocessor-based devices. These processor-based controllers contain, or have access to, inpuLoutput (I/O) interface electronics that allow various types of signals to enter and leave the controller s processor. The controller, depending on its type, uses sufficient read-only-memoiy (ROM) and read/write-accessible-memoiy (RAM) to perform the controller function. 

Currently, the trend in process control is away from centrahzed process control and toward an increased number of small distributed-control or PLC systems. This trend will put emphasis on the evolution of the fieldbus controller and continued growth of the PC-based controller. Also, as hardware and software improves, the functionality of the controller will increase, and the supporting hardware will be physically smaller. Hence, the traditional lines between the DCS and the PLC will become less distinct as systems will be capable of supporting either function set. 

As microprocessor-based controls displaced hardwired electronic and pneumatic controls, the impac t on plant safety has definitely been positive. When automated procedures replace manual procedures for routine operations, the probability of human errors leading to hazardous situations is lowered. The enhanced capability for presenting information to the process operators in a timely manner and in the most meaningful form increases the operator s awareness of the current conditions in the process. Process operators are expected to exercise due diligence in the supervision of the process, and timely recognition of an abnormal situation reduces the likelihood that the situation will progress to the hazardous state. Figure 8-88 depicts the layers of safety protection in a typical chemical jdant. 

In many cases, plants simply live with these problems. However, use of modern model-based control schemes in conjunction with improved methods for on-line moisture and particle size analysis can help overcome these effects [Ennis (ed.), Powder Tech., 82 (1995) Zhang et al., Control of Paiticulate Processes TV (1995)]. 

Drake, E. M. and C. W. Thurston. 1992. A Safety Evaluation Eramework for Process Hazards Management in Chemical Eacilities with PES-Based Controls. Process Safety Management Paper 73d. AlChL 1992 Summer National Meeting, August 1992, Minneapolis, MN. 

With this technology it is now possible to achieve extremely accurate speed control of the order of 0.01 % to 0.001 %. To achieve such high accuracy in speed control, closed-loop feedback control systems and microprocessor-based control logistics can be introduced into the inverter control scheme to sense, monitor and control the variable parameters of the motor to very precise limits. 

Moderate to precise controls as for a.c. drives possible with the use of microprocessor-based control systems... 

All control scheirres shown with auxiliary contactors and timers can be easily replaced with PLCs and microprocessor-based controls. Refer to Section 13.2.3 for mote details. 

To ensure flexibility for possible later additions or changes, the instrumentation has to be selected based on standardized subproducts using national or international norms. These standards are mainly based on the measuring areas and signal types. Typical signals for input and output to the data-based controller (DDC and PTC) are... 

Because of these solutions, important technical data can be transferred from local instrumentation (control system) through data-based controllers to a control station with computers. The operator may use the many variations that the software data system provides. Technical data operation may be digital off/on messages such as the status of operation and the performance of alarms or analog measurements such as temperature, humidity, pressure, velocity, energy usage, etc. 

Digital control A control loop in which a microprocessor-based controller directly controls equipment by means of sensors. Its operation depends on a series of on-off pulses arranged to convey information. 

Mathews and Rawlings (1998) successfully applied model-based control using solids hold-up and liquid density measurements to control the filtrability of a photochemical product. Togkalidou etal. (2001) report results of a factorial design approach to investigate relative effects of operating conditions on the filtration resistance of slurry produced in a semi-continuous batch crystallizer using various empirical chemometric methods. This method is proposed as an alternative approach to the development of first principle mathematical models of crystallization for application to non-ideal crystals shapes such as needles found in many pharmaceutical crystals. 

Microprocessor-based control systems are being increasingly used in place of traditional instrumentation. Some accidents that have occurred on these systems are described in Chapter 20. 

The first set of case studies illustrates errors due to the inadequate design of the human-machine interface (HMI). The HMI is the boundary across which information is transmitted between the process and the plant worker. In the context of process control, the HMI may consist of analog displays such as chart records and dials, or modem video display unit (VDU) based control systems. Besides display elements, the HMI also includes controls such as buttons and switches, or devices such as trackballs in the case of computer controlled systems. The concept of the HMI can also be extended to include all means of conveying information to the worker, including the labeling of control equipment components and chemical containers. Further discussion regarding the HMI is provided in Chapter 2. This section contains examples of deficiencies in the display of process information, in various forms of labeling, and the use of inappropriate instrumentation scales. 

In the new polar organic mode, the ratio of acid/base in the mobile phase affects the selectivity and the concentration of acid and base controls the retention. It is suggested to start the method development with a medium concentration (0.1 %) for both acid and base. If retention is too long or too short, the concentration can be increased to 1 % or reduced to 0.01 %. If no selectivity is observed in this mode, reversed phase is recommended as the next step in the protocols. 

Both electronic and microcomputer-based controls require information about the state of the controlled system. Sensors convert different physical variables into an electric signal that is conditioned and typically converted to a digital signal to be used in microcontrollers. The trend in the construction techniques of modern sensors is the use of silicon microstrnctures because of the good performance and the low cost of this type of device. In the energy control scope the main quantities to be measured are the temperature, pressure, flow, light intensity, humidity (RH), and the electric quantities of voltage and current. 

Manufacturers now offer as standard microprocessor-based control systems as part of their gas turbine generator set. The set can therefore be controlled and monitored from panels adjacent to the machine or, if required, remotely from a central control room. In combined-cycle installations, the system would be linked into the... 

Campbell K, Radcliffe C, Thomas RG, Grundman M, Thai L. Online subject randomization and drug ordering system using the web linked to central data base. Controlled Clin Trials 1999 20 2S 91S. 

Memo. No. 1140. Massachusetts Institute of Technology, Cambridge, MA, 1989. Psichogios, D. C., and Ungar, L. H., Direct and indirect model based control using artificial neural networks, Ind. Eng. Chem. Res. 30, 2564 (1991). 

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